Plasma is generally defined as a collection of charged particles containing about equal numbers of positive ions and electrons and exhibiting some properties of a gas, but differing from a gas in that plasma is generally a good conductor of electricity and may be affected by a magnetic field. A plasma may be generated, for example, by passing a gas through an electric arc. The electric arc will rapidly heat the gas by resistive and radiative heating to very high temperatures within microseconds of the gas passing through the arc. Essentially any gas may be used to produce a plasma in such a manner. Thus, inert or neutral gases (e.g., argon, helium, neon or nitrogen) may be used, reductive gases (e.g., hydrogen, methane, ammonia or carbon monoxide) may be used, or oxidative gases (e.g., oxygen, water vapor, chlorine, or carbon dioxide) may be used depending on the process in which the plasma is to be utilized.
Plasma generators, including those used in conjunction with, for example, plasma torches, plasma jets and plasma arc reactors, generally create an electric discharge in a working gas to create the plasma. Plasma generators have been formed as direct current (DC) generators, alternating current (AC) plasma generators, as radio frequency (RF) plasma generators and as microwave (MW) plasma generators. Plasmas generated with RF or MW sources may be referred to as inductively coupled plasmas. In one example of an RF-type plasma generator, the generator includes an RF source and an induction coil surrounding a working gas. The RF signal sent from the source to the induction coil results in the ionization of the working gas by induction coupling to produce a plasma. In contrast, DC- and AC-type generators may include two or more electrodes (e.g., an anode and cathode) with a voltage differential defined therebetween. An arc may be formed between the electrodes to heat and ionize the surrounding gas such that the gas obtains a plasma state. The resulting plasma, regardless of how it was produced, may then be used for a specified process application.
For example, plasma jets may be used for the precise cutting or shaping of a component; plasma torches may be used in forming a material coating on a substrate or other component; and plasma reactors may be used for the high-temperature heating of material compounds to accommodate the chemical or material processing thereof. Such chemical and material processing may include the reduction and decomposition of hazardous materials. In other applications, plasma reactors have been utilized to assist in the extraction of a desired material, such as a metal or metal alloy, from a compound that contains the desired material.
Processes that utilize plasma-type reactors are disclosed in, for example, U.S. Pat. Nos. 5,935,293 and RE37,853 both issued to Detering et al. and assigned to the assignee of the present invention, the disclosures of each of which patents are incorporated herein in their entireties by this reference. The processes set forth in the Detering patents include the heating of one or more reactants by means of, for example, a plasma torch to form from the reactants a thermodynamically stable high temperature stream containing a desired end product. The gaseous stream is rapidly quenched, such as by expansion of the gas, in order to obtain the desired end products without experiencing back reactions within the gaseous stream. In one embodiment, the desired end product may include acetylene and the reactants may include methane and hydrogen. In another embodiment, the desired end product may include a metal, metal oxide or metal alloy and the reactant may include a specified metallic compound. Of course, such processes are merely examples and numerous other types of processes may be carried out using plasma technologies.
As noted above, process applications utilizing plasma generators are often specialized and, therefore, the associated plasma jets, torches and/or reactors need to be designed and configured according to highly specific criteria. Such specialized designs often result in a device that is limited in its usefulness. In other words, a plasma generator that is configured to process a specific type of material using a specified working gas to form the plasma is not necessarily suitable for use in other processes wherein a different working gas may be required, wherein the plasma is required to exhibit a substantially different temperature or wherein a larger or smaller volume of plasma is desired to be produced.